1. Field of the Invention
The present invention relates to superconducting wires and superconducting coils and in particular, to superconducting wires and superconducting coils which are difficult to quench and are high in reliability and which are used for magnetically levitated trains, nuclear magnetic resonance apparatuses and nuclear fusion apparatuses.
2. Description of Related Art
High purity copper has been exclusively used as stabilizers in conventional superconducting wires. Japanese Patent Kokai No.49-10794 discloses an NbTi wire in which an Al material is provided at the center part for weight-saving and which is covered with Cu. Further, a superconducting wire comprising an NbTi multifilamentary wire as the inner part and an Al stabilizer as the outer peripheral part has also been developed. Japanese Patent Kokoku No.59-6005 discloses a superconducting wire having a stabilizing cover layer comprising a high purity Al alloy containing 0.005-5.0% by weight of alumina.
Furthermore, as mentioned in "Journal of the 91th Japan Mechanical Society", No. 835 (June 1988), pages 36-40, NbTi multifilamentary wires of a low Cu content are used for attaining a high current density of coils in superconducting wires for magnetically levitated trains and recently, wires for MLU002 which have the ratio (ratio of sectional areas of Cu/NbTi) of 1.0 are known. Further known are wires having a large sectional area ratio of (Cu+Al)/NbTi and a small sectional area ratio of Al/Cu.
In the course of the development of the above-mentioned conventional techniques, employment of only the pure Cu has been shifted to that of the high Al ratio (high Al/Cu) because as compared with Cu, Al as a stabilizer has the merits, 1) materials of the higher purity can be used, 2) lighter in weight, 3) smaller in electric resistance in magnetic field, 4) higher in cooling efficiency due to the low specific heat, and 5 higher in transmission of elementary particles. However, use of the higher Al ratio has the problems that a space is apt to occur between the wires in coiling due to the softness of Al and the coil is apt to deform due to the electromagnetic force when excited and as a result the mechanical stability is deteriorated. Moreover, the softness of Al deteriorates the composite extrusion processability of Al with a multifilamentary super-conducting wire of the higher strength.
In addition, in the superconducting wire of Japanese Patent Kokoku No.59-6005, alumina particles cannot be uniformly dispersed and sufficient strengthening cannot be attained.